1. Field of the Invention
The present invention relates to optical sensors and, more particularly, to an optical sensor that measures the light output by the combustion chamber of an internal combustion engine.
2. Description of the Related Art
A spark plug is a well-known part of an internal combustion engine that, when installed, is located in a combustion chamber near the top of a cylinder of the engine. Following the intake and compression cycles of a piston within the cylinder, the spark plug generates a spark across a gap which burns the compressed fuel in the cylinder.
FIG. 1 shows a cross-sectional view that illustrates a prior-art spark plug 100. As shown in FIG. 1, spark plug 100 includes a conductive center electrode 110, a ceramic jacket 112 that fits around center electrode 110, and an outer metal shell 114 that fits around ceramic jacket 112.
Conductive center electrode 110 has a first end 120 and a spaced apart second end 122. In addition, outer metal shell 114 is threaded for insertion into an engine block, and includes a tip 126 that curves up and around to be directly over first end 120 of electrode 110 such that an end 128 of tip 126 is spaced apart from end 120 of electrode 110 by a gap 130.
In operation, center electrode 110, which is connected via end 122 to a rotor or switching device which, in turn, is connected to a coil, periodically receives a voltage spike from the coil via the rotor or switching device. The voltage spike ionizes the air in gap 130 between electrode 110 and tip 126 such that a current flows from electrode 110 to tip 126 (to ground via the engine block) creating a spark.
FIG. 2 shows a time versus voltage graph that illustrates a waveform of an input voltage 200 that is received by a prior art spark plug. As shown in FIG. 2, point A represents the point when current flow in the primary winding of the coil is interrupted. The interruption in current flow in the primary winding induces a current in the secondary winding of the coil which causes voltage 200 to instantaneously spike from point A to point B, and then fall back to point C.
The magnitude of voltage 200 at point B is sufficient to ionize the air and create a current flow (an arch) from electrode 110 to tip 126, while the magnitude of voltage 200 at point C is sufficient to maintain the current flow once it has begun. Following the instantaneous spike, the magnitude of voltage 200 remain substantially constant from point C to point D, where the current flow stops.
The current flow stops when the energy in the second winding of the coil can no longer sustain the current flow. As further shown in FIG. 2, from point D to point E, once the current flow stops, the magnitude of voltage 200 drops and oscillates around ground, eventually settling to ground.
The timing of voltage 200 with respect to the position of the piston head in the cylinder as the piston head nears the completion of the compression cycle impacts the performance of the engine. When the spark plug generates a spark, the completeness of the burn of the fuel mixture depends on the position of the piston head at the time of the spark.
For example, the fuel is burned to a first level when the spark plug generates a spark just before the piston head reaches the end of its stroke, while the fuel is burned to a second level when the spark plug generates a spark as the piston head reaches the end of its stroke. In current generation ignition systems, the timing of the spark (voltage 200) is controlled by an electronic ignition system.